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Mine Tailings

Last Modified: 28th August 2012

Basic Mining Terminology: An overview of basic mining terms and where various components of a mineral deposit end up.

FULL FIGURE

Terminology

Mine tailings are materials left over after extraction of valuable minerals from ore.  They are distinct from “waste rock” that miners move and discard along with soil and organic matter (collectively known as "overburden") as they dig down to access the underlying ore.

The size and composition of the tailings varies depending on the mining method.  In hardrock metal mining the ore is usually crushed, then processed, resulting in very fine particles that may also contain chemicals used for mineral extraction.

Disposal

Disposal of mine tailings is usually the single biggest environmental concern facing a hardrock metal mine. Toxic chemicals used to extract the valuable materials from the ore, such as the cyanide used in gold mining, remain in the tailings at the end of the process, and may leach out into ground water.  Rock may naturally contain dangerous chemicals, such as arsenic and mercury, which leach into water much more readily after rock has been ground up and exposed to the wind and the rain. Acid mine drainage is the most frequent and widespread problem.  Many hardrock mines (including most gold mines) extract minerals that are bound up with sulfide compounds.   These compounds produce sulfuric acid on contact with air and water, a process that occurs at a very low rate in undisturbed rock, at a higher rate in unprotected waste rock (which has a large surface area and is now exposed to air) and a much higher rate in unprotected mine tailings which have a massive surface area.

Typical Mine Tailings Dam

Earthen tailings impoundment dam at Fort Knox gold mine in Alaska

Earthen tailings impoundment dam at Fort Knox gold mine in Alaska

source: Northern Alaska Environmental Center (2004). Copyright held by photographer.

Most modern hardrock metal mines dispose of tailings in pits lined with clay or a synthetic liner.  Many mines put the tailings back into the original mining pit, and some large mines use entire existing valleys or lakes. Disposal pits are usually covered with water, which reduces the rate of sulfuric acid formation.  In many cases, massive dams hold back the tailings and water, requiring long-term oversight and maintenance. The proposed Pebble Mine in Alaska would use such a "tailings impoundment". Some tailings impoundments (e.g. at Red Dog Mine) also require active water treatment of water that flows out of the impoundment.  Both simple and treated impoundments must be maintained in perpetuity in order to prevent the subsequent creation of acid and the release of toxic metals into the environment.

Pogo Mine Tailings Facility

The "dry stack" tailings facility at Pogo Mine, Alaska

The "dry stack" tailings facility at Pogo Mine, Alaska

source: Dave Chambers, CSP2 (2004). Copyright held by photographer.

In "dry-stack" disposal, the tailings are dried and then buried in a covered and lined pit.  This method takes up much less space, is less susceptible to seismic hazards and doesn’t require active water treatment (particularly relevant where precipitation is high).  However, dry-stack disposal is much more expensive… at least in terms of up-front costs.  The other disadvantage of dry-stacking is that the solid tailings must be transported by truck or conveyor instead of by slurry pipeline.  The pit covering must be maintained in perpetuity, but its maintenance requires much less work than a large tailings impoundment with a dam.  In Alaska,  Pogo Mine, Greens Creek Mine, and Nixon Fork Mine use dry-stacking for tailings disposal.

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By David CoilElizabeth LesterBretwood HigmanGround Truth Trekking

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Date Created: 15th December 2010